While vials and ampules have been used as medical containers for a long time, prefilled syringes having an injection container preliminarily filled with a medical liquid have been developed in recent years. Particularly, prefilled syringes made of plastic which are unbreakable, light in weight and excellent in disposability have come to be widely used. The prefilled plastic syringe has a circular tube body, a gasket and a plunger, and is used after an injection needle having a diametric size suited to use is selectively connected to a tip of the circular tube body (in some cases, the injection needle is preliminarily installed). Such a syringe is designed taking into account its air-tightness and liquid-tightness so that the medical liquid contained therein is prevented from evaporating to the outside.
Furthermore, in order to prevent the medical liquid from leaking through a gap between the circular tube body and the gasket under an overwhelming injection resistance at the time of injecting the liquid into a living body, the syringe is designed taking into account also the air-tightness and liquid-tightness in the condition where an internal pressure is exerted.
On the other hand, for such a syringe to function, the gasket must be slid smoothly relative to the above-mentioned circular tube body. Thus, the contact surface is subjected to coating with a lubricant, a surface treatment or the like, if necessary.
As has been mentioned above, the medical containers having a contact surface between a gasket and a circular tube body such as a prefilled syringe are rigorously designed so as to simultaneously fulfill the two contradictory functions, namely, the air-tightness and liquid-tightness, and the slidability of the gasket relative to the circular tube body. However, the currently achievable slidability may be insufficient, in the case where a tiny amount of a medical liquid is injected by use of a syringe pump, or depending on the kind of the medical liquid or the capacity of the syringe.
In view of this, there has been proposed a method for reducing the sliding resistance by reducing the outside diameter of the gasket immediately before use (Patent Documents 1 and 2). According to this method, a medical container capable of reducing the sliding resistance between a syringe outer tube and a gasket immediately before use has been disclosed.
The above-mentioned prior art, however, has problems on the basis of manufacture and use which are yet to be solved. Normally, in the manufacture of a prefilled syringe, a plunger is mounted to a gasket inside a syringe outer tube in a final step. This is because if the plunger were mounted before a filling step, the plunger would obstruct the filling operation. In the case of Patent Document 1, the gasket is pressed against the syringe outer tube by a tip portion of the plunger. Before the plunger is mounted, therefore, the size of a screw hole in the gasket inside the syringe outer tube is necessarily smaller than the screw diameter of the plunger. This results in that it may be difficult to mount the plunger to the gasket in the syringe outer tube. In other words, the manufacturing method for the syringe is limited to a method in which the plunger is mounted to the gasket and thereafter this assembly is inserted into the syringe outer tube.
In addition, the prefilled syringes include those which are sold in the state in which a syringe outer tube portion and a plunger portion are separate from each other, so as to enhance ease of containment of these components into a packaging. In this case, like in the above-mentioned case, because the size of the screw hole in the gasket present inside the syringe outer tube is smaller than the screw diameter of the plunger, again, it is difficult to mount the plunger.
On the other hand, FIGS. 9 and 10 in Patent Document 2 show the use of a slit-rib combination instead of the screw fit. This approach also fails to simultaneously realize operability in inserting the plunger into the gasket and prevention of the plunger from falling off in use. In other words, like in the above-mentioned cases, in order to easily insert the plunger into the gasket present inside the syringe outer tube, there must be a dimensional relation such that the ribs of the plunger are comparatively loosely fitted in the slits formed in the gasket.
After the insertion, the syringe can be stored in the state in which the plunger has been rotated so as to enhance the adhesion between the gasket and the syringe outer tube. When the plunger is rotated in use, the syringe is returned into the state where the adhesion between the gasket and the syringe outer tube is low. Then, the plunger may easily fall off to the hand side because of the loose initial fit between the slits formed in the gasket and the ribs of the plunger.    Patent Document 1: Japanese Patent No. 3211223    Patent Document 2: Japanese Patent No. 3296025